Restarting the hotspot sometimes helps:
If you are not seeing the iPhone hotspot in the Gateway's WiFi network list, sometimes toggling the hotspot off and back on will "wake" the hotspot up and make it broadcast the network again so the Gateway (and other devices) can see it.
Newer iPhones default to 5GHz
Newer iPhones default to 5GHz WiFi for the hotspot functionality. The G1 Gateway needs a 2.4GHz network. On the hotspot page in the iPhone settings, check if there is a toggle near the bottom labeled "Maximize Compatibility" and if so, turn it on and try again.